CVE-2026-21115

Executive Summary

CVE-2026-21115 is an out-of-bounds write vulnerability in the libcodec2-sec-flacdec.so shared library, a secondary FLAC audio decoder shipped on affected Samsung Android platforms. Successful exploitation requires local access to the device and the ability to supply a crafted media file or data stream to the vulnerable decoder path, allowing an attacker to corrupt memory beyond intended buffer boundaries. The flaw is remediated in Samsung Security Maintenance Release Oct-2026 Release 1; until that release is broadly deployed, affected devices remain exposed to local memory-corruption attacks that may escalate to code execution in the context of the media/codec process.

Technical Analysis

The vulnerability class is CWE-787 (Out-of-bounds Write), described in the NVD entry as an "Out-of-bounds write in libcodec2-sec-flacdec.so prior to SMR Oct-2026 Release 1 allows local attackers to write out-of-bounds memory." The affected artifact is a native shared object (.so) — the FLAC decoder component of Samsung's libcodec2-sec codec library family. This places the flaw in the media parsing/decoding subsystem, where attacker-influenced input (audio bitstream data, container metadata, and frame parameters) is processed in native code.

Mechanism (at the vulnerability-class level): out-of-bounds writes in media decoders typically arise when a size, count, offset, or allocation value derived from input is used to index or copy into a destination buffer without adequate bounds validation. FLAC decoding involves reading frame headers, subframe types, residual coding parameters, and block sizes, then reconstructing PCM samples into output buffers. When a length or index field is trusted without validation, the decoder may write past the end of an allocated buffer, corrupting adjacent heap or stack memory.

The NVD description does not disclose which specific input field, function, or code path is at fault, and no CNA title, CWE mapping, CVSS vector, version range, or patch source is provided. Accordingly, the precise trigger, the affected field, and the exact code-level defect cannot be stated with confidence and are not reproduced here. What is clear from the description is: (1) the vulnerable component is libcodec2-sec-flacdec.so; (2) the attacker requires a local vector (SMR = Security Maintenance Release, and the description explicitly states "local attackers"); and (3) the outcome is memory corruption via an out-of-bounds write. The "sec" naming and the involvement of a media codec library suggest the component is designed for secure/privileged media decoding, meaning a memory-corruption primitive here may have elevated impact relative to a sandboxed app process.

Attack surface: applications or services that hand FLAC data to the platform codec library — third-party media players, the system media framework, messaging apps handling received audio, file preview/indexing services, or any interface that processes attacker-supplied FLAC content. Because the trigger is local, the practical distribution vector is usually a malicious file (downloaded, sideloaded, received via messaging, or placed on shared storage) opened by a user or processed by a background media scanner.

Exploitation Scenarios

No public proof-of-concept, exploit code, or Nuclei template exists for CVE-2026-21115, and the NVD entry supplies no specific function names, offsets, or parameters. The following chain is therefore described strictly at the vulnerability-class level and is inferred from the description ("local attackers … out-of-bounds memory write" in a FLAC decoder library); it does not assert implementation details that are not present in the source data.

1. Target identification / reconnaissance: An attacker with local access to a Samsung device (or the ability to deliver a file to one) identifies an application or service that decodes FLAC audio using the platform codec library. The attacker determines whether the target device is running a build prior to SMR Oct-2026 Release 1, which is the vulnerable baseline stated in the NVD description.

2. Trigger: The attacker crafts or obtains a malformed FLAC file/stream designed to exercise the decoder's handling of the input data that leads to the out-of-bounds write. Because the specific malformed field is not disclosed, the trigger is characterized only as input that causes the decoder to compute an out-of-bounds destination offset.

3. Delivery and execution method: Consistent with a local attack vector, the crafted media is delivered by placing the file on device storage or sending it to the victim, then inducing the vulnerable process to decode it — for example, by opening the file in a media application or by relying on an automatic media scanning/preview path. When decoding occurs, the out-of-bounds write corrupts memory adjacent to the intended buffer within the process hosting the decoder.

4. Post-exploitation impact: The immediate result is memory corruption in the decoding process. Depending on which memory is adjacent to the overwritten region (heap metadata, object pointers, control structures) and the privileges of the decoding process, this may lead to a crash (denial of service) or, in the worst case, subversion of the process's control flow. The NVD entry does not state whether the write is reachable pre-authentication, whether the decoder runs in a privileged context, or whether code execution has been demonstrated; those specifics are "insufficient technical detail available" in the provided data.

Impact Assessment

• Confidentiality/Integrity/Availability: An out-of-bounds write is a memory-corruption primitive. Availability impact is near-certain (potential crash/instability when the malformed input is processed). Integrity impact covers the overwritten memory region. Confidentiality impact depends on what data the corrupted structures grant access to. No official CVSS vector or subscores were provided; the scores cited in METADATA are inferred from the vulnerability class (AV:L, AC:L, PR:L, UI:N) and should be treated as provisional.
• Exploitation preconditions: Local access and user/process interaction sufficient to cause decoding of attacker-controlled FLAC content. This is a meaningful barrier, which reduces remote, unauthenticated mass-exploitation risk but does not eliminate targeted or file-delivery-based attacks.
• Blast radius: Limited to devices running affected Samsung builds prior to SMR Oct-2026 Release 1 that expose the libcodec2-sec FLAC decoding path. Each affected device is an independent blast radius; there is no evidence of a network-propagating or supply-chain component.
• Business relevance: Media-decoding memory-corruption bugs are commonly used in targeted mobile attacks and can serve as an initial foothold or sandbox-escape primitive. Where affected devices are issued to high-value personnel, the local-delivery requirement is often satisfied through messaging or web-download lures, raising the practical risk above the nominal "local only" label.
• Score status: CVSS is N/A in the provided data. EPSS is not yet scored by FIRST.org and must not be assumed. CISA KEV status is NO, so there is no agency confirm of known exploitation.

Detection Guidance

Sigma Rule

title: Potential CVE-2026-21115 Media Decoder Memory Corruption Activity
id: 9f1c7b2a-21115-4c0e-9a11-000000021115
status: experimental
description: Detects abnormal termination or crash activity involving media codec processes that may indicate exploitation of the out-of-bounds write in libcodec2-sec-flacdec.so (CVE-2026-21115).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-21115
author: valtersit.com
date: 2026/10/01
logsource:
  category: process_creation
  product: linux
detection:
  selection_process:
    Image|endswith:
      - '/mediaserver'
      - '/media.codec'
      - '/audioserver'
  selection_loader:
    CommandLine|contains:
      - 'libcodec2-sec-flacdec.so'
      - 'libcodec2-sec'
  condition: selection_process or selection_loader
falsepositives:
  - Normal media playback and codec loading on Samsung devices
level: medium
tags:
  - attack.t1203
  - cve.2026.21115

YARA Rule

rule CVE_2026_21115_libcodec2_sec_flacdec
{
    meta:
        description = "Identifies the vulnerable FLAC decoder shared object associated with CVE-2026-21115"
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-21115"
        date = "2026-10-01"
        severity = "medium"

    strings:
        $lib = "libcodec2-sec-flacdec.so" ascii wide
        $flac = "FLAC" ascii wide
        $codec = "codec2" ascii wide nocase
        $flacmagic = { 66 4C 61 43 } // "fLaC" stream marker

    condition:
        (uint16(0) == 0x457F or uint32(0) == 0x464C457F) and
        (any of ($lib, $codec)) and
        any of ($flac, $flacmagic)
}

Suricata IDS Rule

alert tcp $HOME_NET any -> $HOME_NET any (msg:"CVE-2026-21115 Possible Malformed FLAC in Transfer to Affected Decoder"; flow:established,to_server; content:"fLaC"; offset:0; depth:4; content:"|00 00 00 00 00 00|"; within:64; classtype:attempted-user; sid:202621115; rev:1; reference:cve,2026-21115; metadata:service http, file_data;)

Elastic Detection

process where host.os.type == "linux" and
  process.name in ("mediaserver", "audioserver", "media.codec") and
  (process.command_line : "*libcodec2-sec-flacdec.so*" or process.command_line : "*libcodec2-sec*")

Splunk SPL Query

index=main (sourcetype=android_log OR sourcetype=syslog OR sourcetype=linux_secure)
("libcodec2-sec-flacdec" OR "flacdec" OR "CVE-2026-21115" OR ("SIGSEGV" AND "codec"))
| stats count min(_time) as firstSeen max(_time) as lastSeen by host, process, message
| where count > 0
| sort - count

Wazuh OSSEC Rule

<group name="cve-2026-21115,codec,memory_corruption,">
  <rule id="20261115" level="10">
    <decoded_as>json</decoded_as>
    <field name="message">libcodec2-sec-flacdec|flacdec|out-of-bounds|SIGSEGV</field>
    <description>CVE-2026-21115: Possible exploitation of FLAC decoder out-of-bounds write in libcodec2-sec-flacdec.so</description>
    <mitre>
      <id>T1203</id>
    </mitre>
    <group>cve_2026_21115,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). No vendor fix, advisory, affected-version list, or workaround is currently recorded in the provided intelligence.

1. Immediate mitigations (compensating controls):

   • Treat SMR Oct-2026 Release 1 as the remediation target and confirm eligibility per-device; until the fix is available on a device, restrict processing of untrusted FLAC content on that device.
   • Reduce the local attack surface: instruct users not to open unsolicited audio files or media attachments on affected devices; disable or restrict background media indexing/preview where configurable.
   • Where MDM/UEM is available, apply policy to block sideloaded/untrusted media playback apps and, if feasible, blocklist or quarantine the vulnerable library path from non-system callers.
   • Monitor for crashes in media/codec processes as an indicator of attempted exploitation (see detection rules above).

2. Patch/upgrade steps:

   • Apply the vendor Security Maintenance Release once it is distributed (target: SMR Oct-2026 Release 1 or later). Track the official Samsung Mobile Security bulletin for the exact release, affected model list, and fixed build numbers, none of which are provided in the current data.
   • Because patch confidence is 0/4, do not assume any currently available build is fixed; verify the firmware security patch level on each device (Settings ▸ About phone ▸ Software information) before treating a device as remediated.

3. Long-term hardening:

   • Integrate Android security patch level tracking into asset inventory and enforce a maximum patch-age policy for mobile fleets.
   • Require memory-safe or hardened media decoding for untrusted content where alternatives exist, and isolate high-risk media handling in low-privilege sandboxes.
   • Subscribe to vendor SMR bulletins and NVD updates for CVE-2026-21115 to obtain the authoritative CVSS vector, CWE mapping, affected-version list, and any exploit evidence that emerges.

References

• NVD — CVE-2026-21115: https://nvd.nist.gov/vuln/detail/CVE-2026-21115
• Samsung Mobile Security (Security Maintenance Releases / bulletins): https://security.samsungmobile.com/
• CWE-787 Out-of-bounds Write — MITRE: https://cwe.mitre.org/data/definitions/787.html
• CISA Known Exploited Vulnerabilities Catalog (check current KEV status): https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• FIRST EPSS (score pending — verify before use): https://www.first.org/epss/
• Threat actor / exploit-in-the-wild evidence: none currently associated with CVE-2026-21115 in the provided intelligence; PoC status is None and no CISA KEV listing is present.